1. Field of the Invention
This invention relates in general to oil well drilling equipment, and in particular to interference warning devices for preventing collisions between components of an oil well drilling rig.
2. Description of the Prior Art
In a typical oil well drilling rig, a traveling block and hook are suspended from the derrick crown block by a series of cables which are driven by the derrick drawworks to raise and lower the traveling block along the vertical axis of the drilling rig. A block retractor is provided, in some cases, to retract the traveling block away from the vertical axis of the derrick, or to extend the traveling block over the vertical axis. A pair of links and a derrick elevator are suspended from the hook, and support lengths of drill pipe or casing as the lengths of drill pipe or casing are added to or removed from the drill string. As the traveling equipment moves up and down along the vertical axis of the derrick, there is a danger of collision between the traveling equipment and other equipment which extends over the vertical axis of the derrick.
Racker arms, attached to the side of the derrick, extend over the vertical axis of the derrick to grip lengths of drill pipe or casing and to move the lengths between the vertical axis of the derrick and a pipe rack offset from the vertical axis. When the racker arm is extended over the vertical axis of the derrick, and the block retractor has extended the traveling equipment over the vertical axis, there is a possibility of collision. Various interference warning devices have been devised to warn the derrick operator of an impending collision. These warning devices emit a single warning when two pieces of equipment have been moved into a danger area. Since one or both pieces of equipment may be moving at a rather high rate of speed, the time lapse between the warning and the collision may be quite small, giving the operator little reaction time in which to prevent the collision or to minimize the damage.
The sensors used in prior art warning devices have been electronic sensors, such as electronic limit switches. These are disadvantageous, because it is desirable to limit electrical circuits attached to moving equipment in the drilling rig.